1. Field of the Invention
The invention relates to a quadrature modulation circuit. In particular, the present invention relates to a quadrature modulation circuit which includes a base band wave reshaping circuit used for digital modulation such as four phase shift keying modulation (QPSK) in which the frequency band is limited by the digital transmission system.
2. Description of the Prior Art
FIG. 8 shows a block diagram of a conventional quadrature modulation circuit used for QPSK. In FIG. 8, an in -phase channel signal 1.sub.i (I-ch) and a quadrature-phase channel signal 1.sub.q (Q-ch) are non-return-to-zero (NRZ) input signals. Low pass filters (ROM LPF) 2.sub.i and 2.sub.q are read only memories (ROM) respectively which operate as band limitation filters for I-ch and Q-ch. Digital to analog converters (D/A converter) 3.sub.i and 3.sub.q convert the digital signals which are received from the ROM LPF 2.sub.i and ROM LPF 2.sub.q, to analog signals. Analog filters 4.sub.i and 4.sub.q suppress the step aliases received from the D/A converters 3.sub.i and 3.sub.q. A quadrature modulation circuit 5 which includes a phase shifter 51, multipliers 52, 53 and an adder 54 modulate a carrier orthogonally with the output signals of the analog filters 4.sub.i and 4.sub.q. An oscillator 6 supplies the modulation carrier signal to the quadrature modulation circuit 5.
FIG. 9 shows a block diagram of the low pass filters (ROM LPF) 2.sub.i and 2.sub.q in FIG. 8. In FIG. 9, an input signal 1 corresponds to the in-phase channel signal 1.sub.i (I-ch) and the quadrature channel signal 1.sub.q (Q-ch). An n-step shift register 21 shifts the input signal 1 in sequence. An oscillator 22 generates a clock signal corresponding to the sample frequency of the ROM LPF 2.sub.i and 2.sub.q. A ROM 24 stores the resulting data of the wave form from the filter.
FIG. 10 shows another block diagram of the low pass filters (ROM LPF) 2.sub.i and 2.sub.q in FIG. 8. A pair of n/2-step shift registers 211 and 212 shift the first half cycle of the input signal and the second half cycle of the input signal respectively. ROMs 241 and 242 store the different wave forms. An adder 25 adds the values received from the ROMs 241 and 242.
The operation of the above conventional art is explained hereinafter. In the digital modulation, such as QPSK, since the frequency component spreads over a wide range, the frequency of the modulated output signal is limited by band limitation filter. A QPSK signal S(t) limited in the base band frequency is expressed in the following equation (1). ##EQU1## where .omega..sub.c is a carrier frequency, I.sub.k and Q.sub.k are the digital signals of I-ch and Q-ch and have the value of +1 or -1 and h (t) is the impulse response of the band limitation filter. A nyquist filter having the characteristics of a raised-cosine roll-off is used for the band limitation filter.
The operation of FIG. 8 is explained by referring the equation (1). An in-phase channel signal 1.sub.i (I-ch) and a quadrature channel signal 1.sub.q (Q-ch) are inputted to the low pass filters (ROM LPF) 2.sub.i and 2.sub.q respectively by the form of NRZ signal I.sub.k and Q.sub.k. Input signals I.sub.k and Q.sub.k are convoluted to form impulse responses in the low pass filters (ROM LPF) 2.sub.i and 2.sub.q respectively. Smoothed wave forms I (t) and Q (t) are outputted as sampled and quantized numerical data from the low pass filters (ROM LPF) 2.sub.i and 2.sub.q respectively. These output data are inputted to the D/A converters 3.sub.i and 3.sub.q respectively and converted into analog signals. The analog filters 4.sub.i and 4.sub.q smooth the step data converted to the analog signals, suppress the aliases generated at the sampling process, and the output signals I (t) and Q (t) are inputted to the quadrature modulator 5. In the quadrature modulator 5, the carrier signal generated in the generator 6 is distributed into two quadrature carriers -sin .omega..sub.c and cos .omega..sub.c which is shifted 90 degrees using a shifter 51. These two carrier signals are applied to multipliers 52 and 53 and are multiplied by the output signals I (t) and Q (t) received from the analog filter 4.sub.i and 4.sub.q respectively. The two outputs from the multipliers 52 and 53 are added in an adder 54 and are outputted as a modulation wave form S (t).
The operations of the ROMs LPF 2.sub.i and 2.sub.q are explained by using FIG. 9 and FIG. 11. The operation of the LPF can be considered as the convolution of the input signal and the impulse response of the LPF. Therefore, they are expressed as the second and the third equations of equation (1).
FIG. 11 shows the convolution result of the equation (1). In FIG. 11, numeral 7 shows input impulse row (I.sub.k or Q.sub.k). The upward arrow shows "1" and downward arrow shows "0". 8 is an impulse response wave form [I.sub.k .multidot.h(t-kT) or Q.sub.k .multidot.h(t-kT)] of the LPF for each input impulse 7. These impulse response wave forms [I.sub.k .multidot.h(t-kT) or Q.sub.k .multidot.h(t-kT)] are shown in dotted lines. 9 is a filter output wave from [I (t) or Q (t)] in which all impulse response wave forms are added. The filter output wave form [I (t) or Q (t)] is shown in solid line.
The range k of .SIGMA. is k=-.infin. to .infin.. As easily known from each impulse response wave form 8 in FIG. 11, the value of the impulse response becomes negligibly small where .vertline.k.vertline. is very large. Therefore, the impulse response can be restricted within the finite range. In this example, 5 symbols before and 5 symbols after a certain symbol (total symbols are 10) are used for calculating the convolution of the impulse response. In this case, the impulse response wave form between the "5" symbol and "6" symbol shown in the solid line is calculated using 10 symbols shown in FIG. 11. When the convolution is calculated from the finite impulse response, the filter output wave form I (t) or Q (t) is obtained as the summation of all impulse response wave forms corresponding to each 10 symbols. That is, the impulse response wave form between the "5" symbol and "6" is calculated from only 10 symbols of "1" to "10" symbols.
FIG. 9 shows a ROM LPF which includes the ROM 24 for storing the wave form described above. In FIG. 9, digital signals I.sub.k or Q.sub.k (input signal 1) are inputted to the n-step shift register 21. The shift register 21 shifts the input data (symbol) in sequence and stores the most recent n symbols and outputs these n symbols to the address of the ROM 24. In this embodiment, as the 10 symbols are used, n is equal to 10.
All combination wave forms of n symbols are calculated beforehand and stored in the ROM 24. In this case, the wave form can not be processed continuously on the time axis. Therefore, the wave forms between two symbols are sampled on the time point of 2.sup.m and the quantized data is stored in the ROM 24. The m bits output from the 2.sup.m binary counter 23 which operates at the sampling clock received from the oscillator 22 is inputted to the ROM 24 as well as the n symbols received from the shift register 21. The ROM LPF in FIG. 9 operates as the LPF by selecting the output wave form stored in the ROM 24 at a time according to the address data constructed of n symbol data received from the shift register 21 and by reading in sequence the b 2.sup.m sampling number between the two symbols which is selected according to the output value from the counter 23.
The capacity of the ROM 24 is decided by the referred symbol data n and the sampling number 2.sup.m between the two symbols. For example, in the case of QPSK, as I.sub.k and Q.sub.k are expressed by one bit respectively, if n=10 and m=3, then the necessary capacity for the ROM 24 is 2.sup.(n+m) =2.sup.13 =8K words respectively for each I-ch and Q-ch ROM. Further, if n becomes larger in order to make the truncation error of the impulse response smaller, the capacity of the ROM will be increasing exponentially.
FIG. 10 is a block diagram of ROMs LPF 2.sub.i and 2.sub.q configuration which is able to decrease the required capacity of the ROM 24 of FIG. 9. In FIG. 10, the operation of the low pass filter is modified, and expressed by equation (2) which is introduced from the second and third equations of equation (1) as follows. ##EQU2##
In equation (2), the range of the impulse response exists between finite n symbols.
The operation of the FIG. 10 is explained using FIG. 12 and equation (2). The filter output wave form is considered as the summation of the filter output wave forms shown in FIGS. 12(a) and (b). That is, the wave form of the FIG. 12(a) indicates the first term of the right side of equation (2) and FIG. 12(b) indicates the second term of the right side of the equation (2). The reference numbers 71.about.91 and 72.about.92 in FIG. 12 correspond to the number 7.about.9 in FIG. 11.
The wave forms shown in FIG. 12(a) and (b) are stored in a ROM 241 and 242 of FIG. 10 respectively in the same way as stored in the ROM 24 in FIG. 9. Each n/2 data from the shift registers 211 and 212 and m bit data from the counter 23 are inputted to the ROMs 241 and 242 respectively, and the corresponding data are read from the ROMs 241 and 242 respectively. The two output data from the ROM 241 are added in an adder 25. That is, the ROM 241 operates to calculates the first term of the right side of equation (2), the ROM 241 operates to calculates the second term of the right side of equation (2), and the adder 25 calculates the addition of the right side of equation (2).
The n data input is divided into two portions, and the first half n/2 data (k=-n/2.about.-1) is stored in the n/2 step shift register 211 and the second half n/2 data (k=0.about.n/2 -1) is stored in the n/2 step shift register 212. The address data from the shift register 211 is outputted to the ROM 241, and the address data from the shift register 212 is outputted to the ROM 242. The m bits output from the counter 23 is inputted to the both ROM 241 and 242. The operation of the m bits output is the same as explained in the FIG. 9.
The capacity of the ROMs 241 and 242 in FIG. 10 is calculated as follows. For example, if n=10 and m=3, then the capacity of the both ROMs is 2.sup.(n/2+m) .times.2=2.sup.8 .times.2=512 words. In the case of 16 QAM, 8 PSK and .pi./4 shifted DQPSK, the capacity of the ROM is 2.sup.(2.times.n/2+m) .times.2=2.sup.13 .times.2=16K words.
As discussed above, the capacity of the ROM of FIG. 10 becomes smaller than that of FIG. 9. But, the capacity of the ROMs 241 and 242 still occupies a considerable amount of memory in the quadrature modulation circuit of FIG. 8. It is also necessary to provide two sets of the same ROM in the quadrature modulation circuit for each I-ch and Q-ch.
There is prior art, for example, laid-open Japanese patent publication No. 63-77246/1988, which describes such quadrature modulation.
As the conventional quadrature modulation is constructed as discussed above, it is necessary to provide a large capacity ROM LPF 2.sub.i and ROM LPF 2.sub.q for each I-ch and Q-ch respectively.
It is a primary object of the present invention to provide a quadrature modulation circuit which requires small capacity ROMs for operating as filters.
It is a further object of the present invention to reduce the hardware size compared with the prior art quadrature modulation circuit having ROMs for operating as filters.
It is a further object of the present invention to reduce the ROM size by using the amplitude symmetry of the wave form.
It is a still further object of the present invention to reduce the ROM size by using the symmetry of the wave form on the time axis.